This invention relates to an improved method and apparatus for the precise measurement of rainfall from low to heavy rainfall rates. In particular, it is an improvement over existing U.S. Pat. No. 4,520,667 by Nelson. Following are the specific details regarding the improved features over the prior art.
The raingauge under Pat. No. 4,520,667 is a drop based raingauge that utilizes the following distinct features;
1. One filter that serves only to filter the rainwater collected and which does not provide a pressure drop.
2. The collector cross sectional area is designed so that the maximum rate of rainwater collected does not exceed the maximum rate of drops being discharged. In other words the rate in equals the rate out.
3. A deflector and sloping water transfer means is required to minimize the rainwaters momentum and thus ensure drop volume uniformity.
4. The detection of a drop, via the probe matrix, is transmitted back to the receiving end by simply providing the resistance of the drop itself.
The main premise on which the aforementioned invention operates is that each drop being discharged is uniform in volume from low to heavy rainfall rates. Although this invention did provide an improvement over the prior art, there are limitations inherent to this approach, detailed as follows:
1. The resolution, or the number of drops detected per each one hundreth of an inch of rainfall received, is limited; to ensure that the drop discharge raate does not reach a point to where the volume of each drop does not begin to significantly increase and thus introduce an error.
2. The fact that the rate out equals the rate in means that as the rainfall rate itself varies, which many times does vary widely in relatively short time frames, the drop discharge rate will follow making methods of determining the drop volume, based upon the discharge rate, unstable and error prone.
3. The calibration process necessary to determine the actual nominal drop size, would require that the fixed volume of water being added to the collector be added at a slow fixed rate so as to not affect the drop size. This then requires additional apparatus to control the rate of water being added to the collector during this process.
4. The method of transmitting the detection of a drop to the receiving end by supplying the drop resistance means that the signal line used to carry this signal is a high impedance line, and thus is susceptible to electrical noise. Also, there is no means to eliminate the effects of the static electricity may have upon the drop detection means.
The present invention incorporates means to overcome the limitations as detailed above. The first item listed, which describes the limit on measurement resolution, is eliminated in the present invention by the use of the vent tube/reservoir combination. A collector cross sectional area could be chosen so as to allow the receipt of a significant volume of water, and thus a high number of drops per each one hundred of an inch, while at the same time implementing a reservoir to ensure that the water level existing within the collector and reservoir remain low enough so as to both maintain an acceptable drop discharge rate, and to avoid to overflow of the collector.
The second item listed as a limitation deals with the unpredictable fluctuations in the discharge rate if wide variations in the actual rainfall rate is encountered. Due to the filter pressure drops incorporated in the present invention, water is allowed to accumulate in the collector/reservoir area, which means that for the drop discharge rate to vary the water height in the collector must vary, which in turn is directly proportional to the water volume. Therefore, for the drop discharge rate to appreciably vary, a significant amount of rainfall must be received, which in turn produces a characteristic in the present invention where even though the discharge rate is allowed to vary over a predetermined range, this variance is "smooth" and predictable in its transistion.
The third item regarding the earlier patents limitations deals with the calibration process. In the present invention, due to its inherent characteristic of allowing water to accumulate in the collector/reservoir prior to being discharged, the calibration process is greatly simplified in that the fixed volume of water to be placed into the collector, that's necessary to perform the process, can be added instantaneouly, without the need for an additional external apparatus. This fact also lends the present invention to being easily calibrated by an automated "electronic calibration" approach.
The fourth item of the prior art as previously described deals with the means of transmitting the drop detection to the receiving signal processor. This approach renders the signal line that carries drop detectiion susceptible to electrical noise, due to the high impedance characteristics of the signal line, which may introduce errors to signal processor if the signal line is routed through an electrically noisy environment. The means of creating a current loop by using the drop resistance to directly turn a switching transistor on, in conjunction with a low value load resistor the receiving end of the signal transmission, produces a signal line characteristic that is low in impedance, and thus highly immune to electrical noise interference; such as would be expected during an electrical storm. Also, with the implementation of a means to discharge any static electricity that may have accumulated in the rain water, the integrity of the transmitted signal is further enhanced.
To summarize the specific improvements of the present invention over the prior art then would be; one, that the present invention allows a high degree of resolution with maintained accuracy; two, that the present invention allows variations in drop discharge rates, which are smooth and predictable in their transitions, and subsequently then that said variations can be used in computing both the actual drop volume as well as performing estimations of rainwater received but not yet discharged; three, that the present invention's design lends itself to being easily calibrated in electronic and automated fashion; and four, that the means of transmitting the drop detection to the signal processor, is more reliable in its electrical integrity, due to its immunity to both static electricity and electrical noise.